Cellulose products such as absorbent sheets and other structures are composed of cellulose fibers which, in turn, are composed of individual cellulose chains. Commonly, cellulose fibers are crosslinked to impart advantageous properties such as increased absorbent capacity, bulk, and resilience to structures containing cellulose fibers. High-bulk fibers are generally highly crosslinked fibers characterized by high absorbent capacity and high resilience.
Crosslinked cellulose fibers and methods for their preparation are widely known. Tersoro and Willard, Cellulose and Cellulose Derivatives, Bikales and Segal, eds., Part V, Wiley-Interscience, New York, (1971), pp. 835-875. Most commonly, the term "crosslinked cellulose fiber" refers to a cellulose fiber having intrafiber crosslinks, i.e., crosslinks between individual cellulose chains within a single cellulose fiber. Generally, intrafiber crosslinks are formed by curing a crosslinking agent in the presence of the fibers. "Curing" refers to covalent bond formation (i.e., crosslink formation) between the crosslinking agent and the fiber. Crosslinking agents are generally bifunctional compounds, and in the context of cellulose crosslinking, these agents covalently couple a hydroxy group of one cellulose chain to another hydroxy group on a neighboring cellulose chain. Many crosslinking agents have been utilized in cellulose crosslinking achieving varying degrees of success.
Common cellulose crosslinking agents include aldehyde and urea-based formaldehyde addition products. See, for example, U.S. Pat. Nos. 3,224,926; 3,241,533; 3,932,209; 4,035,147; and 3,756,913. While these crosslinking agents have been widely used in some environments, their applicability to absorbent products that contact human skin (e.g., diapers) has been limited by safety concerns. These crosslinkers are known to cause irritation to human skin. Moreover, formaldehyde, which persists in formaldehyde-crosslinked products, is a known health hazard and has been listed as a carcinogen by the EPA. Accordingly, the disadvantages associated with formaldehyde and other formaldehyde-derived crosslinking agents has prompted the development of safer alternatives.
Other aldehyde crosslinking agents are also known. For example, dialdehyde crosslinking agents (i.e., C.sub.2 -C.sub.8 dialdehydes and preferably glutaraldehyde) have also been utilized in the production of absorbent structures containing crosslinked cellulose fibers. See, for example, U.S. Pat. Nos. 4,689,118 and 4,822,453. While these dialdehyde crosslinkers appear to overcome the health risks associated with formaldehyde crosslinkers, these crosslinking agents suffer from commercial disadvantages related to the costs of producing dialdehyde crosslinked fibers.
Cellulose has also been crosslinked by carboxylic acid crosslinking agents. Certain polycarboxylic acids have been used to provide absorbent structures that have the polycarboxylic acid reacted with fibers in the form of intrafiber crosslink bonds. For example, U.S. Pat. Nos. 5,137,537, 5,183,707, and 5,190,563 describe the use of C.sub.2 -C.sub.9 polycarboxylic acids crosslinking agents. These C.sub.2 -C.sub.9 polycarboxylic acids are low molecular weight polycarboxylic acids that contain at least three carboxyl groups, and have from two to nine carbons in the chain or ring separating two of the carboxyl groups. Exemplary C.sub.2 -C.sub.9 polycarboxylic acids include 1,2,3-tricarboxypropane, 1,2,3,4-tetracarboxybutane, and oxydisuccinic acid. A particularly preferred C.sub.2 -C.sub.9 polycarboxylic acid is 2-hydroxy-1,2,3-tricarboxypropane, also known as citric acid. Unlike the aldehyde-based crosslinking agents noted above, these polycarboxylic acids are nontoxic and, for the preferred polycarboxylic acid citric acid, the crosslinking agent is commercially available at relatively low cost. Moreover, while the aldehyde-based crosslinking agents form relatively unstable acetal crosslinked bonds, C.sub.2 -C.sub.9 polycarboxylic acid crosslinking agents provide relatively stable ester crosslinks.
While some of the disadvantages associated with the crosslinking agents noted above have been overcome by the development and utilization of new and improved crosslinking agents, crosslinking agents are generally characterized by their relatively narrow cure temperature range. The length of time at a particular cure temperature is also a factor in crosslinking fibers. The narrow cure temperature range of traditional crosslinking agents, such as those noted above, is due to their chemical reactivity. Most crosslinking agents have bifunctional reactivity and will undergo crosslinking at a temperature sufficient to cause the functional groups of the crosslinking agent (e.g., the aldehyde group of formaldehyde, or a carboxylic acid group of citric acid) to react with crosslink sites of the cellulose fiber (i.e., a hydroxy group). Generally, crosslinking occurs rapidly once a temperature sufficient to effect bond formation between the agent and fibers is reached.
Accordingly, there is a need in the art for a crosslinking agent that allows for greater flexibility in the production of crosslinked fibers having specific, desirable properties. More specifically, there exists a need for a safe and economical crosslinking agent curable over a wide temperature range to provide crosslinked fibers having a correspondingly wide range of crosslinking and associated advantageous absorbent properties.
Despite the advantages that polycarboxylic acid crosslinking agents provide, certain crosslinked cellulosic fibers, particularly cellulosic fibers crosslinked with low molecular weight polycarboxylic acids such as citric acid, tend to lose their crosslinks over time and revert to uncrosslinked fibers. For example, citric acid crosslinked fibers show a considerable loss of crosslinks on storage. Such a reversion of crosslinking generally defeats the purpose of fiber crosslinking, which is to increase the fiber's bulk and capacity. Thus, the useful shelf-life of fibers crosslinked with these polycarboxylic acids is relatively short and renders the fibers somewhat limited in their utility.
The loss of crosslinking results in a loss of the advantageous properties imparted to the fibers by crosslinking. Aged fibers, that is, fibers that have been subject to crosslinking reversion, can be characterized as having relatively lower bulk, diminished absorbent capacity, and lower liquid acquisition capability compared to the same fibers as originally formed.
Accordingly, there exists a need for stable intrafiber crosslinked cellulose fibers that offer the absorbent properties and advantages afforded by traditional crosslinked fibers and that also retain their intrafiber crosslinks over time and in storage to provide a crosslinked fiber having a substantial useful shelf-life. The present invention seeks to fulfill these needs and provides further related advantages.